The present invention relates generally to the field of frequency synthesizer circuits, and more particularly to the field of citizens band (CB) transceiver frequency synthesizer circuits in which the synthesizer supplies both the receiver and transmitter injection frequencies for tuning the citizens band transceiver.
A typical radio receiver synthesizer configuration is illustrated in U.S. Pat. No. 4,135,158 owned by the assignee of the present invention. An example of a typical CB transceiver synthesizer configuration is illustrated in U.S. continuation patent application Ser. No. 005,544, filed Jan. 22, 1979, and assigned to the same assignee as the present invention. Both of these prior art synthesizer configurations do not provide for minimizing the cost of the synthesizer while maintaining high performance characteristics. Both of these prior art synthesizers utilize phase-frequency locked loops with voltage controlled oscillators (VCO) and electronically controlled programmable frequency dividers to produce the desired receiver and/or transmitter injection frequencies. In this respect both of the prior art references are similar to the present invention.
The U.S. Pat. No. 4,135,158 contemplates the use of substantially dividing down in frequency the output of the voltage controlled oscillator in order to provide a suitable input for a programmable frequency divider. While this minimizes the cost of the programmable frequency divider by providing a relatively low frequency input to the programmable divider, this requires the use of a relatively expensive fixed divider to divide down the output of the voltage controlled oscillator. The U.S. continuation patent application referred to above avoids the use of this relatively costly fixed divider by utilizing a difference mixer which mixes the output of the voltage controlled oscillator with another high frequency signal to produce a low frequency difference signal that serves as the input to the programmable frequency divider. The continuation application, however, unlike the present invention, utilizes a relatively costly multiplier stage to step up in frequency the output signal of a master crystal oscillator so as to provide a proper high frequency input signal to the difference mixer which also receives the VCO output as an input. The use of this multiplier stage increases the cost of the continuation application synthesizer and also creates undesired spurious signals which will degrade performance for receivers using this synthesizer.
In addition to the two prior art circuits mentioned above, it should be noted that a prior art frequency synthesizers exists (see U.S. Pat. No. 4,061,973) in which one crystal oscillator is used to provide an input signal to the difference mixer for mixing with the VCO output signal while a different crystal oscillator is used as a reference oscillator whose output, after appropriate frequency division, provides a reference signal as an input to a phase-frequency comparator which receives the output of a programmable frequency divider that receives the output of the difference mixer. In this prior art circuit, the cost of the two separate crystal oscillators provides a distinct cost disadvantage for this configuration, and the fact that two separate oscillators are used leads to synthesizer stability problems since the oscillators have the potential for varying in frequency and phase in different manners.